This invention relates to methods of making partially foamed extruded plastic products, viz. products having a foamed core layer of resin and a substantially unfoamed skin layer contiguous and bonded thereto.
One illustrative use of the present method is in producing multilayer sheet plastic stock for making disposable dinner plates and the like. The stock has a central core layer of foamed or expanded resin sandwiched between and bonded to unexpanded thin skin layers of resin, the foamed core layer importing desired bulk to the product while the skin layers protect the core against surface abrasion and cutting, contribute to product stiffness, and provide an attractive surface appearance.
Conveniently, the stock is produced by coextruding the three layers as concentric strata of a tube or hollow cylindrical sleeve which is slit into sheets beyond the coextrusion die. Coextrusion is accomplished by forcing concurrent, contiguous, concentric flows (skin/core/skin) of resin in a fluent, extrudable state, herein termed a melt, through an annular orifice defined between a circular die aperture and a cylindrical mandrel surface. The core-forming flow has a foamable composition, so as to expand into a predominantly closed cellular structure after release from the pressure within the die at the temperature at which extrusion occurs, and the skin-forming flows are at least substantially nonfoamable, such that the skin layers after extrusion have a relatively dense, noncellular structure. Ultimately, the produced sheet stock is cut into plates or other desired articles, leaving a substantial amount of scrap which is advantageously recycled to constitute a supply of resin for re-extrusion to produce further sheet stock.
A foamed structure of extruded thermoplastic resin is produced by incorporating in the resin a suitable proportion of a blowing agent which, at a sufficiently elevated temperature and at atmospheric pressure, provides gas for creating a multiplicity of cells or voids that expand the resin layer or body into a foam that stabilizes upon subsequent cooling. The blowing agent may be a chemical agent, which decomposes when heated to produce the foam-creating gas, or a physical agent (e.g. a hydrocarbon, such as butane or pentane), which undergoes a phase change from liquid to gas when heated. Physical blowing agents, in general, are capable of providing a much greater reduction in density of a resin layer or body than are chemical blowing agents. Typically, a physical blowing agent is employed in conjunction with a nucleating agent, dispersed through the body or layer of resin to be foamed; when the resin, advancing through an extrusion die at elevated temperature and pressure, is released to atmospheric pressure by emergence from the die, the liquid blowing agent vaporizes and forms gas bubbles at the nucleating sites provided by the dispersed nucleating agent to create the desired foamed structure of voids or cells throughout the resin.
Various systems and procedures are known for incorporating blowing and nucleating agents in a resin melt. U.S. Pat. No. 3,787,542, for instance, describes a procedure in which a resin melt is divided into plural streams to facilitate mixing; a first blowing agent is added to all the streams; a second blowing agent, which serves as a nucleating agent for the first blowing agent, is then added to all the streams; and the streams are recombined for extrusion through a die. In another, conventional foam extrusion process, resin pellets are mixed with a nucleating agent and melted in a heating extruder in which the plastic and nucleating agent are held under high temperature and pressure; to the pressurized melt is added a liquid physical blowing agent, which acts as a plasticizer, reducing the melt viscosity and lowering the level of temperature necessary to maintain the hot melt condition of the mixture of plastic and nucleating agent. This mixture, with the blowing agent added, is cooled to an extrusion temperature suitable for foaming, and extruded through a die, becoming foamed upon vaporization of the blowing agent when released to atmospheric pressure beyond the die. One particular system uses polystyrene as the plastic, pentane as the blowing agent, and a mixture of citric acid and sodium bicarbonate as the nucleating agent.
The systems and procedures just mentioned are employed for production of articles having a single, uniformly foamed layer. To make a product having a foamed core layer and an unfoamed skin layer or layers in a coextrusion die, it has heretofore been proposed to employ the same resin in both the core-forming and the skin-forming flows in the die and to incorporate a blowing agent only in the core-forming flow. For example, U.S. Pat. Nos. 3,782,870 and 3,956,438 describe procedures wherein a resin melt is established in a heating extruder which divides the melt into two streams, for advance to a coextrusion die, and adds a blowing agent to only one of these streams. Owing, however, to the viscosity-lowering effect of physical blowing agents, the contiguous flows of resin in the die (one flow containing, and the other lacking, a blowing agent), though both constituted of the same resin, differ from each other in viscosity; and this difference in viscosity tends to cause flow instability and undesired mixing of the core-forming and skin-forming flows within the die. The effects of viscosity mismatch in a coextrusion die can be mitigated, in a coextrusion die system wherein only the core-forming flow contains a blowing agent, by using different resins for the core-forming and skin-forming flows respectively, with the skin-forming resin selected to have an inherently lower viscosity than the core-forming resin; but the lower-viscosity resins tend to have inferior physical peroperties (e.g. brittleness), and in addition, the presence of layers of two different resins (with respectively different viscosities) in the extruded product hinders or prevents recycling of product scrap as a source of resin for either layer.
Some other proposals for making extruded plastic products with contiguous foamed and unfoamed layers, while using the same resin for both layers, have involved physically separated rather than contiguous flows of core-forming and skin-forming melts, and have thus necessitated use of special equipment characterized by undesirable structural and/or operational complexity as compared with ordinary coextrusion dies.
A solution to many of the foregoing difficulties is provided by the method described and claimed in U.S. Pat. No. 4,518,557, issued May 21, 1985, to Sheldon M. Wecker, one of the applicants herein. In this method, the same resin is used for both the core-forming and skin-forming flows, which are advanced contiguously through a coextrusion die, and all flows contain the same blowing agent; the blowing agent in the skin-forming flow or flows acts as a plasticizer so that the viscosity of such flow or flows is comparable to that of the core-forming flow. Foaming action is restricted to the core layer by incorporating a nucleating agent only in the core-forming melt (the skin-forming melt being thus at least substantially free of nucleating agent), and extruding at a temperature at which the blowing agent causes foaming only in the presence of a nucleating agent. As the patent explains, for various blowing agents (especially physical blowing agents), there is a range of temperatures at which the blowing agent will cause foaming if but only if a nucleating agent is present, although at higher temperatures the blowing agent will cause foaming even without a nucleating agent. Consequently, by appropriate mutual selection of blowing agent, nucleating agent and extrusion temperature, the presence of nucleating agent only in the core layer (and not in the skin layer or layers) will produce foaming action restricted to the core layer notwithstanding that the blowing agent is present in both core and skin.
In specific embodiments of the method of the last-mentioned patent, as therein described, the core-forming melt is prepared by supplying resin and nucleating agent to the hopper of a first heating extruder with introduction of a blowing agent (e.g. pentane) at a downstream locality therein, and the skin-forming melt is prepared by supplying resin without nucleating agent to the hopper of a second heating extruder, again with introduction of blowing agent at a downstream locality therein. Thus, two separate heating extruders are used, from which the respective melts are separately advanced to a coextrusion die where they are brought together in continguous flows.